Carbon nanotubes can be classified by the number of walls in the tube, single-wall, double wall and multiwall. Each wall of a carbon nanotube can be further classified into chiral or non-chiral forms. Carbon nanotubes are currently manufactured as agglomerated nanotube balls or bundles. Use of carbon nanotubes as a reinforcing agent in polymer composites is an area in which carbon nanotubes are predicted to have significant utility. However, utilization of carbon nanotubes in these applications has been hampered due to the general inability to reliably produce individualized carbon nanotubes. To reach the full potential of performance enhancement of carbon nanotubes as composites in polymers the aspect ratio, that is length to diameter ratio, should be substantially greater than 40. The maximum aspect ratio for a given tube length is reached when each tube is fully separated from another. A bundle of carbon nanotubes, for example, has an effective aspect ratio in composites of the average length of the bundle divided by the bundle diameter.
Various methods have been developed to debundle or disentangle carbon nanotubes in solution. For example, carbon nanotubes may be shortened extensively by aggressive oxidative means and then dispersed as individual nanotubes in dilute solution. These tubes have low aspect ratios not suitable for high strength composite materials. Carbon nanotubes may also be dispersed in very dilute solution as individuals by sonication in the presence of a surfactant. Illustrative surfactants used for dispersing carbon nanotubes in aqueous solution include, for example, sodium dodecyl sulfate, or cetyltrimethyl ammonium bromide. In some instances, solutions of individualized carbon nanotubes may be prepared from polymer-wrapped carbon nanotubes. Individualized single-wall carbon nanotube solutions have also been prepared in very dilute solutions using polysaccharides, polypeptides, water-soluble polymers, nucleic acids, DNA, polynucleotides, polyimides, and polyvinylpyrrolidone. The dilution ranges are often in the mg/liter ranges and not suitable for commercial usage.
Surfaces with electrically conductive properties are widely distributed in economic applications, for example in the manufacture of electrical switching circuits, sensors and heating coils.
In this context, the traces are applied on to the surface by means of various processes. It is common to the known products, however, that the resulting conductive properties are based on metallic or semiconductive coating materials.
Essential to the aforementioned products is their generally high degree of distribution. The materials and processes used must therefore enable the resulting component to be produced at the lowest possible costs in order to meet the high demand cheaply. Processes that make this possible are e.g. the common screen-printing processes for the production of electrically conductive coatings.
This requirement leads to the fact that the use of metallic conductors, particularly of precious metals, on components in some areas of application is disadvantageous, particularly from the point of view of price. Applications that have become known fairly recently are, for example, so-called “Radio Frequency Identification” tags (RFID tags for short). These are passive or active electronic components which are substantially used for the storage and transfer of data relating to the object on which they are located.
Studies exist according to which, in 2008 in Europe alone, out of 260 billion individual products, as many as 5% (i.e. 13 billion) are said to be fitted with one of these components. (Press release, “Enorme Wachstumsraten far RFID-Markt in Europa” [“Enormous growth rates for RFID market in Europe”], SOREON Research GmbH, Frankfurt am Main, 10 May 2004).
Among other things it is conceivable that, for many of these products, the component is applied to a package which must be disposed of after the product it contains has been used. Consequently, metallic conductors or semiconductor products are disadvantageous in disposal since they are difficult to incinerate completely. On the other hand, components largely consisting of readily combustible substances would offer an advantage here. Suitable examples of these would be conductive pastes or inks based on carbon black or graphite, or the special carbon nanotubes presented in this invention.
A prerequisite for the good electrical conductivity of the coatings is a fine dispersion of the conductive particles in the formulations used for the coating in each case and a high specific conductivity thereof.
In U.S. Pat. App. Pub. No. US 2006/124028 A1, an ink is disclosed for such a purpose which employs carbon nanotubes for use in ink jet printers. The ink is characterized by a surface tension of 0.02-0.07 N/m and a viscosity of 0.001-0.03 Pa·s at 25° C. The content of carbon nanotubes is disclosed within broad limits as 0.1-30 wt. %. The inks are not suitable for screen printing, with a viscosity of up to 0.03 Pa·s. A viscosity of the order of magnitude of 1 Pa·s would be needed for this purpose.
In U.S. Pat. App. Pub. No. US 2005/284232 A1, an electrically conductive coating which contains carbon nanofibres is disclosed. The coating is intended to be applied by brushing, rolling or spraying an appropriate ink. The use of the ink for screen printing is not disclosed. The ink has a content of carbon nanofibres of 4-12 wt. % in a matrix similar to the substrate, here for example urethanes, polyimides, cyanate esters and other organics. No disclosure is given relating to the parameters relevant to screen printing, such as, e.g., surface tension on a certain substrate or viscosity. It is disclosed that the viscosity can be reduced by dissolving the matrix.
In International Pat. Pub. No. WO 2005/119772 A2, an ink is disclosed comprising carbon nanotubes, wherein the carbon nanotubes used have an external diameter of no more than 20 nm and are used in a concentration of ≤10 wt. %. The post-treatment temperature is disclosed as greater than 75° C., and this should last for at least 10 minutes. In addition, compositions of an ink for use e.g. in screen printing are disclosed, which use derivatives of cellulose, among other things, to achieve or obtain dispersion in the resulting formulation. The resulting surface resistance of the inks after treatment according to the disclosure is a maximum of 10 kΩ/m.
In International Pat. Pub. No. WO 2005/029528 A1, inks or pastes comprising carbon nanotubes are disclosed, which are applied on to surfaces by various printing techniques (e.g. screen printing) for the purpose of producing electrodes. The inks disclosed are either aqueous formulations comprising carbon nanotubes with inorganic auxiliary agents, or formulations in organic solvents comprising carbon nanotubes with organic, polymeric auxiliary agents. The carbon nanotubes used are the types generally known to the person skilled in the art. The physical properties of the inks with respect to viscosity, surface tension and conductivity are not disclosed. The inks disclosed are disadvantageous since they are either present in organic solvents and thus are potentially an environmental risk, or they comprise inorganic auxiliary agents, such as Al2O3 or SiO2, which are non-conductive and are also not easy to remove in the course of a post-treatment. It can therefore be assumed that the conductivity of the printed image is disadvantageous compared with an ink without these auxiliary agents.
In the prior art set out above, carbon nanotubes of the cylinder type are always used for the production of inks. These carbon nanotubes are structures of either single wall (so-called “single wall carbon nanotubes”—SWNTs—) or multiwall (so-called “multi wall carbon nanotubes”—MWNTs—) carbon nanotubes, as described e.g. in the publication by Ijima (publication: S. Ijima, NATURE Vol. 354, pp. 56-58, 1991). These known carbon nanotubes are characterized by structures of carbon tubes in which one or more closed, concentrically arranged graphene layers form the basis of the structure of the nanotubes.